Model comparisons - tested September 2026 by Vitus Pruser. How we test.
Verdict
Both finished all ten tasks with zero hallucinations, so this came down to the blind ranking, where GPT-6 Astra took 8 of 10 for answers that stuck closer to the brief. Claude Fable 5.1 won the EU VAT question and the jug puzzle, was faster on coding and data, and added more context at roughly twice the cost. Pick GPT-6 Astra when you want exactly what you asked for, Claude when you want the explanation around it.
Verdict by task
| Task | Winner | Why |
|---|
| Writing | GPT-6 Astra | Hit the 120 word target exactly and kept every fact in the rewrite; Claude ran slightly long and added an unrequested subject line and tip. |
| Coding | GPT-6 Astra | Ranked first on both tasks for the leaner solutions, although Claude was correct on both, five times faster on slugify, and its off by one cent fix avoids a third party dependency. |
| Research | Split | Both were fully correct on both questions. The panel preferred Claude on the EU VAT threshold (notes that national thresholds vary) and GPT-6 Astra on the population ranking (shorter, sourced). |
| Data analysis | GPT-6 Astra | Both got every number right. GPT-6 Astra ranked first for a cleaner presentation; Claude added a month by month table and compound growth, which the panel found more than the prompt asked for. |
| Reasoning | Split | Both solved both puzzles. Claude won the water jug puzzle by proving the 6 step route is the minimum; GPT-6 Astra won the bill split with the shortest correct answer. |
Results, prompt by prompt
writing - winner: GPT-6 Astra
Prompt: Write a 120 word product update email to existing customers of a small accounting software company announcing that bank feeds now sync every hour instead of once a day. Plain, friendly, no exclamation marks, end with one clear call to action.
- Claude Fable 5.1 - pass, no hallucinations, 36.1s. 127 words including subject and sign-off (body about 120). Adds a subject line and a manual refresh tip that was not in the brief; slightly outside the 120 word target.
- GPT-6 Astra - pass, no hallucinations, 24.1s. Exactly 120 words, no exclamation marks, one CTA. Plain and friendly as asked.
writing - winner: GPT-6 Astra
Prompt: Rewrite this paragraph so a 12 year old understands it, keep every fact, max 80 words: 'Compound interest is interest calculated on the initial principal and also on the accumulated interest of previous periods. Because the base grows each period, the growth is exponential rather than linear, which is why long time horizons matter far more than the interest rate alone.'
- Claude Fable 5.1 - pass, no hallucinations, 8.4s. 72 words, all facts kept, snowball image makes it the most 12 year old friendly.
- GPT-6 Astra - pass, no hallucinations, 8.4s. 66 words, keeps all facts including exponential versus linear and the principal plus accumulated interest definition. Reads a bit technical for a 12 year old.
coding - winner: GPT-6 Astra
Prompt: Write a Python function slugify(title: str) -> str that lowercases, replaces any run of non alphanumeric characters with a single hyphen, strips leading and trailing hyphens, and transliterates common accented Latin letters (e, a, o, u with accents, and the German sharp s) to ASCII. Include 5 pytest test cases. No external dependencies.
- Claude Fable 5.1 - pass, no hallucinations, 19.2s. Correct, handles ss, ae, oe and o slash explicitly, commented, 5 descriptive tests. All own tests and all 8 extra cases pass. Fastest at 19 seconds.
- GPT-6 Astra - pass, no hallucinations, 102.5s. Shortest correct solution (unicodedata NFD). Own 5 tests pass, plus 8 of our extra transliteration cases. Slow: 102 seconds, most of it reasoning.
coding - winner: GPT-6 Astra
Prompt: This JavaScript has a bug: users report the total is sometimes off by one cent. Find the bug, explain it in two sentences, and give the corrected function.
function total(items) {
let sum = 0;
for (const it of items) {
sum += it.price * it.qty * (1 + it.taxRate);
}
return Math.round(sum * 100) / 100;
}
- Claude Fable 5.1 - pass, no hallucinations, 13.3s. Correct diagnosis, fix works in integer cents with per line rounding and no dependency, and explains the invoice subtotal alternative.
- GPT-6 Astra - pass, no hallucinations, 14.5s. Correctly identifies binary floating point rounding (1.005 * 100 = 100.49999999999999). Fix pulls in decimal.js, a dependency the prompt did not ask for.
research - winner: Claude Fable 5.1
Prompt: What is the current EU VAT registration threshold for small businesses under the EU wide SME scheme that started in 2025, in euros, and which regulation introduced it? Answer in 3 sentences and say if you are unsure about any figure.
- Claude Fable 5.1 - pass, no hallucinations, 17s. Correct on all figures and instruments, notes that national thresholds vary. Longest of the three answers for a 3 sentence request.
- GPT-6 Astra - pass, no hallucinations, 13.3s. Correct: 100,000 EUR Union threshold, 85,000 EUR national cap, Council Directive (EU) 2020/285 and Regulation 2020/283, 1 January 2025. Flags that it is a directive, not a regulation. Checked against the directive text.
research - winner: GPT-6 Astra
Prompt: List the 5 largest countries by population in 2024 with their approximate population, and name the source you would cite for these figures. Table format.
- Claude Fable 5.1 - pass, no hallucinations, 14.8s. Same figures and source, adds alternative sources and the India overtook China in 2023 note, which is correct per the UN.
- GPT-6 Astra - pass, no hallucinations, 12.4s. Ranking and figures match UN World Population Prospects 2024 mid-2024 estimates, source named with link.
data - winner: GPT-6 Astra
Prompt: Here are monthly revenue figures in EUR for a shop: Jan 41200, Feb 38900, Mar 45100, Apr 47800, May 46200, Jun 52300, Jul 55900, Aug 51400, Sep 58700. Compute the month over month growth for each month, the average growth rate, and the best and worst month. Show the numbers in a table and state the totals.
- Claude Fable 5.1 - pass, no hallucinations, 101.2s. All numbers verified correct, adds average monthly revenue (48,611) and compound growth (4.52 percent), both correct. 101 seconds.
- GPT-6 Astra - pass, no hallucinations, 101.2s. All eight growth rates, 4.90 percent average, 437,500 total, best and worst by revenue and by growth all verified correct. 101 seconds.
data - winner: GPT-6 Astra
Prompt: A SaaS company has 1,240 customers paying an average of 49 EUR per month and loses 3.1 percent of customers each month. New signups are 85 per month. Estimate the customer count and MRR after 12 months assuming these rates hold. Show your working and round to whole numbers.
- Claude Fable 5.1 - pass, no hallucinations, 24.1s. Same result plus a month by month table and the 2,742 steady state, verified. The alternative assumption figure (about 1,690 customers, 82.8k) is slightly off: exact is 1,686 and 82,609 EUR, within its stated rounding.
- GPT-6 Astra - pass, no hallucinations, 42.2s. Closed form recurrence, 1,713 customers and 83,920 EUR MRR, verified (1,712.65 and 83,919.76). Uses LaTeX, which the Arena renders.
reasoning - winner: Claude Fable 5.1
Prompt: I have a 3 litre jug and a 5 litre jug and unlimited water. Give the shortest sequence of steps to measure exactly 4 litres. Then say how many steps it took.
- Claude Fable 5.1 - pass, no hallucinations, 14.4s. Correct 6 steps with jug states, and correctly states the alternative route takes 8 steps (verified).
- GPT-6 Astra - pass, no hallucinations, 10.8s. Correct 6 step sequence, correct count. Most concise.
reasoning - winner: GPT-6 Astra
Prompt: Three colleagues need to split a 187 EUR dinner. Anna had no drinks and her food was 41 EUR. Ben and Clara shared two bottles of wine totalling 58 EUR and their food was 44 EUR each. Who owes what, so that everyone pays exactly for what they consumed? Show the check that the parts add up to 187.
- Claude Fable 5.1 - pass, no hallucinations, 9.6s. Correct, two independent checks.
- GPT-6 Astra - pass, no hallucinations, 7.2s. 41, 73, 73, check to 187. Correct and shortest.
Frequently asked questions
Is Claude or ChatGPT better?
In our September 2026 round, GPT-6 Astra ranked first on 8 of 10 prompts in a blind review, while Claude Fable 5.1 won the EU VAT research question and the water jug puzzle. Both completed every task with zero hallucinations, so the difference is style: GPT-6 Astra answers the brief and stops, Claude explains more and costs about twice as much per answer.
Which is better for coding, Claude or ChatGPT?
Both produced correct, tested code on our two coding prompts. The panel ranked GPT-6 Astra first for leaner solutions, but Claude Fable 5.1 was much faster (19 seconds versus 102 on the slugify task) and its rounding fix needed no extra library, so for day to day coding the gap is small.
Which is cheaper to run, Claude or ChatGPT?
Measured in Krater across the ten prompts, GPT-6 Astra cost $0.23 in total and Claude Fable 5.1 $0.44. Claude was cheaper on the short rewrite prompt and more expensive on everything else.
Can I use Claude and ChatGPT in the same place?
Yes. Krater gives you Claude Fable 5.1, GPT-6 Astra and 500+ other models in one workspace, and the Model Arena lets you send one prompt to several models side by side, which is how this comparison was run.
How was this comparison tested?
All three models (GPT-6 Astra, Claude Fable 5.1 and DeepSeek V4 Pro) received the same ten prompts side by side in the Krater Model Arena on 16 September 2026, with default settings and no system prompt. We recorded time to a complete answer and the model cost per answer, fact checked every claim against a primary source, scored task completion as pass, partial or fail, and then Malte and Vitus Pruser ranked the answers blind, without knowing which model wrote which. The full procedure is on our methodology page.
Will this comparison be updated?
Yes. Flagship comparisons are rerun monthly and whenever a provider ships a new version. The tested month and model versions are shown at the top of the page.
